1. Field of the Invention
The present invention relates to a blood pressure measuring apparatus and, more particularly, to a blood pressure measuring apparatus for oppressing arteries and measuring the blood pressures on the basis of the obtained results of the arterial volume changes.
2. Description of the Related Art
Methods for measuring blood pressures without giving stress include a method of detecting various pulsation signals (hereinafter, referred to as pulse waves) derived from volume changes of arteries to which pressure is applied from outside, in the process of varying the pressure gradually, and calculating and determining the blood pressures on the basis of the detected values (the method being referred to as oscillometric method). In a representative method, air or fluid is injected into an oppression band (cuff) which is wound around the blood pressure measuring position, and the arteries in the measuring position are oppressed. Pulse waves are detected as pulsations of oppressing pressure, that is, the cuff pressure, and this method is mostly widely employed (hereinafter, referred to as cuff oscillometric method) (see, for example, Japanese Unexamined Patent Publication No. 5-31084 (1993), specification and drawings and Japanese Unexamined Patent Publication No. 6-133938 (1994), specification and drawings).
In the cuff oscillometric method, pulse waves are extracted from cuff pressure signals, and in other oscillometric method, for example, other pulse wave detector (for example, photoelectric sensor) is additionally installed. Alternatively, an oscillometric method may be realized by oppressing arteries by other oppressing function than the cuff. All these varieties are called the oscillometric method in the following description.
A method of detecting the blood pressure at a certain time is disclosed in Japanese Published Patent Publication No. 2000-512875, pages 30–31, FIG. 27, and FIG. 29A to FIG. 29C. In this method, by obtaining the arterial volume signal when the cuff pressure is equal to or less than the diastolic pressure and the arterial volume signal when the cuff pressure is somewhere between the diastolic pressure and systolic pressure, the two obtained arterial volume signals are compared by matching the start timing; therefore, the moment of coincidence of cuff pressure and arterial pressure is known.
In the oscillometric method, for example, when measuring while increasing the cuff pressure, as shown in FIG. 12, blood pressures (systolic pressure and diastolic pressure) are estimated from the envelopes showing amplitude change patterns of pulse waves changing in the process of gradual elevation of the cuff pressure until sufficiently higher than the maximum pressure (systolic pressure). Although not shown in the drawing, when measuring while decreasing the cuff pressure, the cuff pressure is once raised quickly sufficiently higher than the systolic pressure, and is gradually decreased until sufficiently lower than the minimum pressure (diastolic pressure), and in this process, similarly, the blood pressures are estimated from the envelopes of pulse waves. In such estimating method, a high pressure level and a long measuring time are required according to its principle. A high pressure level often causes pain in a patient with hypertension. Pain during measurement of blood pressure is not only unpleasant, but also causes to raise the blood pressure, thereby causing effects on accuracy of blood pressure measurement. In addition, if one measurement takes a long time, not only the efficiency is lowered, but also the pain lasts for a long time, and the accuracy is sacrificed. Further, sudden blood pressure fluctuations often occurring during measurement or measurement during exercise cannot be correctly detected.